Ni-Based Heat Resistant Alloy and Method for Producing the Same

ABSTRACT

A Ni-based heat resistant alloy of the present invention contains predetermined amounts of C, Si, Mn, P, S, N, O, Ni, Co, Cr, Mo, W, B, Al, Ti, Nb, REM, Mg, Ca, and the balance of Fe and impurities, wherein [0.1≤Mo+W≤12.0], [1.0≤4×Al+2×Ti+Nb≤12.0], and [P+0.2×Cr×B&lt;0.035] are satisfied, a shortest distance from a center portion to an outer surface portion of a cross section of an alloy member is 40 mm or more, the cross section being perpendicular to a longitudinal direction of the alloy member, an austenite grain size number at the outer surface portion is −2.0 to 4.0, a total content of Al, Ti and Nb which are present as precipitates obtained by extraction residue analysis satisfies [(Al+Ti+Nb) PB /(Al+Ti+Nb) PS ≤10.0], and [YS S /YS B ≤1.5] and [TS S /TS B   ≤1.2 ] are satisfied at a normal temperature.

TECHNICAL FIELD

The present invention relates to a Ni-based heat resistant alloy and a method for producing the same.

In recent years, ultra super critical boilers, where temperature and pressure of steam are increased, have been newly installed worldwide so as to enhance efficiency. With respect to these ultra super critical boilers, there are plans to increase the steam temperature, which is conventionally about 600° C., to 650° C. or above, or further to 700° C. or above. Accordingly, the development of techniques has been underway in Japan and abroad.

This development is based on the fact that energy saving, effective use of resources, and reduction in CO₂ gas emissions for environmental preservation have been some of the tasks to be solved with respect to energy problems, and such development is an important industrial policy. Further, the development has been underway because, in the case of power generation boilers which burn fossil fuels, reactors for chemical industry and the like, it is advantageous to use ultra super critical boilers and reactors which have high efficiency.

Increasing temperature and pressure of steam causes the temperature of superheater pipes of boilers and pipes of reactors for chemical industry and, further, thick plates and forged products, which are used as heat resistant pressurized parts, to rise to 700° C. or above during the actual operation. Accordingly, the alloy used in such a severe environment for long periods of time is required to possess not only high temperature strength and high temperature corrosion resistance, but also excellent stability of metal micro-structure for long periods of time, excellent creep rupture ductility, and excellent creep fatigue resistance.

In terms of the above strict requirements, a Fe-based alloy, such as austenitic stainless steel, has insufficient creep rupture strength. Accordingly, it is inevitable to use a Ni-based alloy which utilizes the precipitation of γ′ phase or the like. Further, welding is inevitably applied to pipes for boilers and chemical industrial plants and hence, the alloy is also required to possess excellent weldability.

As an alloy which satisfies the above strict requirements, for example, the Patent Document 1 discloses an austenitic heat resistant alloy which is excellent in both of weld crack resistance and toughness in a heat affected zone (HAZ), and is also excellent in creep strength at a high temperature.

LIST OF PRIOR ART DOCUMENTS Patent Document

Patent Document 1: JP4697357B

SUMMARY OF INVENTION Technical Problem

Large-sized structural members made of a material for apparatuses, such as boilers or chemical plants, are hot rolled or hot forged and then subjected to final heat treatment without cold rolling before putting into use. Accordingly, the grain size is relatively large. For this reason, usually, there is a problem that 0.2% proof stress and tensile strength at a normal temperature, which are defined as the specifications of materials, are lower than those of a material obtained by performing final heat treatment after cold rolling.

In addition to the above, in a large-sized structural member, a cooling speed at the time of performing heat treatment varies largely from region to region and hence, there is a variation from region to region in the amount of solid solution elements which contribute to strengthening the member as precipitates during use at a high temperature. There is also a problem that creep rupture strength varies due to such variation. Accordingly, it is difficult to adopt alloy disclosed in Patent Document 1 to a large-sized structural member.

The present invention has been made to overcome the above problems, and an objective of the present invention is to provide a Ni-based heat resistant alloy and a method for producing the same which exhibits sufficient 0.2% proof stress and tensile strength at a normal temperature, and sufficient creep rupture strength at a high temperature in large-sized structural members.

Solution to Problem

The present invention has been made to overcome the above problems, and the gist of the present invention is the following Ni-based heat resistant alloy and method for producing the same.

(1) A Ni-based heat resistant alloy having a chemical composition consisting of, in mass %:

C: 0.005 to 0.15%;

Si: 2.0% or less;

Mn: 3.0% or less;

P: 0.030% or less;

S: 0.010% or less;

N: 0.030% or less;

O: 0.030% or less;

Ni: 40.0 to 60.0%;

Co: 0.01 to 25.0%;

Cr: 15.0% or more to less than 28.0%;

Mo: 12.0% or less;

W: less than 4.0%;

B: 0.0005 to 0.006%;

Al: 0 to 3.0%;

Ti: 0 to 3.0%;

Nb: 0 to 3.0%;

REM: 0 to 0.1%;

Mg: 0 to 0.02%;

Ca: 0 to 0.02%; and

the balance: Fe and impurities, wherein following formulas (i) to (iii) are satisfied,

a shortest distance from a center portion to an outer surface portion of a cross section of the alloy is 40 mm or more, the cross section being perpendicular to a longitudinal direction of the alloy,

an austenite grain size number at the outer surface portion is −2.0 to 4.0,

a total content of Al, Ti and Nb which are present as precipitates obtained by extraction residue analysis satisfies a following formula (iv), and

mechanical properties at a normal temperature satisfy a following formula (v) and a following formula (vi):

0.1≤Mo+W≤12.0   (i)

1.0≤4×Al+2 ×Ti+Nb≤12.0   (ii)

P+0.2×Cr×B<0.035   (iii)

(Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS)≤10.0   (iv)

YS_(S)/YS_(B)31.5   (v)

TS_(S)/TS_(B)≤1.2   (vi)

wherein, symbol of an element in the formulas (i) to (iii) refers to content (mass %) of each element, and meaning of each symbol in the formulas (iv) to (vi) is as follows:

(Al+Ti+Nb)_(PB): total content of Al, Ti and Nb which are present at center portion as precipitates obtained by extraction residue analysis

(Al+Ti+Nb)_(PS): total content of Al, Ti and Nb which are present at outer surface portion as precipitates obtained by extraction residue analysis

YS_(B): 0.2% proof stress at center portion

YS_(S): 0.2% proof stress at outer surface portion

TS_(B): tensile strength at center portion

TS_(S): tensile strength at outer surface portion.

(2) The Ni-based heat resistant alloy described in the above (1), wherein

the chemical composition contains one or two elements selected from a group consisting of, in mass %:

Mg: 0.0001 to 0.02%; and

Ca: 0.0001 to 0.02%. (3) The Ni-based heat resistant alloy described in the above (1) or (2), wherein

10,000-hour creep rupture strength at 700° C. in the longitudinal direction at the center portion is 150 MPa or more.

(4) A method for producing a Ni-based heat resistant alloy, the method including the steps of:

performing hot working on an ingot or a cast piece having the chemical composition described in the above (1) or (2); and

thereafter performing heat treatment where the ingot or the cast piece is heated to a heat-treatment temperature T (° C.) ranging from 1070 to 1220° C., is held for 1150 D/T to 1500 D/T (min), and is cooled with water,

wherein symbol “D” denotes a maximum value (mm) of a linear distance between an arbitrary point on an outer edge of a cross section of the alloy and another arbitrary point on the outer edge, the cross section being perpendicular to a longitudinal direction of the alloy.

(5) The method for producing a Ni-based heat resistant alloy described in the above (4), wherein

in the step of performing the hot working, the hot working is performed one or more times in a direction substantially perpendicular to a longitudinal direction in the hot working.

Advantageous Effects of Invention

The Ni-based heat resistant alloy of the present invention has small variation in mechanical properties from region to region, and is excellent in creep rupture strength at a high temperature.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the respective requirements of the present invention are described in detail.

1.Chemical Composition

The reasons for limiting respective elements are as follows. In the description made hereinafter, symbol “%” for content refers to “mass %”.

C: 0.005 to 0.15%

C (carbon) makes the austenitic structure stable, and forms fine carbides at grain boundaries, thus enhancing creep strength at a high temperature. Accordingly, it is necessary to set a content of C to 0.005% or more. However, when the C content is excessively increased, carbides are coarsened, and a large amount of carbides precipitates and hence, ductility at the grain boundaries is lowered, thus causing lowering of toughness and creep strength. Accordingly, the C content is set to 0.15% or less. The C content is preferably 0.01% or more. Further, the C content is preferably 0.12% or less, and more preferably 0.10% or less.

Si: 2.0% or less

Si (silicon) is contained as a deoxidizing element. Further, Si is an element effective in enhancing corrosion resistance and oxidation resistance at a high temperature. However, when a content of Si exceeds 2.0%, stability of the austenite phase is lowered, thus causing lowering of toughness and creep strength. Accordingly, the Si content is set to 2.0% or less. The Si content is preferably 1.5% or less, and more preferably 1.0% or less. It is not particularly necessary to set the lower limit of the Si content. However, when the Si content is excessively reduced, deoxidation effect cannot be sufficiently obtained, thus deteriorating cleanliness of the alloy, and causing an increase in production cost. Accordingly, the Si content is preferably 0.02% or more, and more preferably 0.10% or more.

Mn: 3.0% or less

Mn (manganese) is an element which has a deoxidizing action in the same manner as Si, and which contributes to stabilization of austenite. However, when a content of Mn exceeds 3.0%, embrittlement occurs so that toughness and creep ductility are lowered. Accordingly, the Mn content is set to 3.0% or less. The Mn content is preferably 2.5% or less, more preferably 2.0% or less, and further preferably 1.5% or less. It is not necessary to set the lower limit of the Mn content. However, when the Mn content is excessively lowered, deoxidation effect cannot be sufficiently obtained, thus deteriorating cleanliness of the alloy, and causing an increase in production cost. Accordingly, the Mn content is preferably 0.02% or more, more preferably 0.10% or more, and further preferably 0.15% or more.

P: 0.030% or less

P (phosphorus) is contained in the alloy as an impurity. However, P is an element which segregates at the crystal grain boundary of the HAZ during welding, thus increasing liquation cracking susceptibility, and adversely affecting toughness after long-term use. For this reason, it is preferable to reduce a content of P as much as possible. However, excessive reduction of the P content causes an increase in steel production cost. Accordingly, the P content is set to 0.030% or less. The P content is preferably 0.020% or less.

S: 0.010% or less

S (sulfur) is contained in the alloy as an impurity. However, S is an element which segregates at the crystal grain boundary of the HAZ during welding, thus increasing liquation cracking susceptibility, and adversely affecting toughness after long-term use. For this reason, it is preferable to reduce a content of S as much as possible. However, excessive reduction of the S content causes an increase in steel production cost. Accordingly, the S content is set to 0.010% or less. The S content is preferably 0.005% or less.

N: 0.030% or less

N (nitrogen) is an element effective in making the austenite phase stable. However, within the range of Cr content in the present invention, excessively large N content causes a large amount of fine nitrides to precipitate within grains during use at a high temperature, thus causing lowering of creep ductility or toughness. Accordingly, a content of N is set to 0.030% or less. The N content is preferably 0.020% or less, and more preferably 0.015% or less. It is not particularly necessary to set the lower limit of the N content. However, excessive reduction of the N content causes an increase in production cost. Accordingly, the N content is preferably 0.0005% or more, more preferably 0.001% or more, and further preferably 0.005% or more.

O: 0.030% or less

O(oxygen) is contained in the alloy as an impurity. However, excessively large O content causes lowering of hot workability, and deterioration of toughness and ductility. Accordingly, a content of O is set to 0.030% or less. The O content is preferably 0.020% or less, more preferably 0.010% or less, and further preferably 0.005% or less. It is not particularly necessary to set the lower limit of the O content. However, excessive lowering of the O content causes an increase in production cost. Accordingly, the O content is preferably 0.001% or more.

Ni: 40.0 to 60.0%

Ni (nickel) is an element effective in obtaining the austenitic structure, and is an indispensable element for ensuring structural stability after long-term use. Further, Ni is bonded to Al, Ti, and Nb, thus forming the fine intermetallic compound phase and hence, Ni also has an action of increasing creep strength. To sufficiently obtain the advantageous effects of Ni within the range of the Cr content in the present invention, it is necessary to set a content of Ni to 40.0% or more. However, Ni is an expensive element and hence, if the Ni content exceeds 60.0%, cost is increased. Accordingly, the Ni content is set to 40.0 to 60.0%. The Ni content is preferably 42.0% or more, more preferably 45.0% or more, and further preferably 48.0% or more, and the Ni content is preferably 58.0% or less.

Co: 0.01 to 25.0%

Co (cobalt) is an austenite former in the same manner as Ni. Co increases stability of the austenite phase, thus contributing to enhancing creep strength. To obtain such an advantageous effect, it is necessary to set a content of Co to 0.01% or more. However, Co is an extremely expensive element and hence, if the Co content exceeds 25.0%, cost is significantly increased. Accordingly, the Co content is set to 0.01 to 25.0%. The Co content is preferably 0.1% or more, more preferably 2.0% or more, and further preferably 8.0% or more. Further, the Co content is preferably 23.0% or less, and more preferably 21.0% or less.

Cr: 15.0% or more to less than 28.0%

Cr (chromium) is an indispensable element for ensuring oxidation resistance and corrosion resistance at a high temperature. To obtain the advantageous effects of Cr within the range of the Ni content in the present invention, it is necessary to set a content of Cr to 15.0% or more. However, when the Cr content becomes 28.0% or more, stability of the austenite phase at a high temperature deteriorates, thus causing lowering of creep strength. Accordingly, the Cr content is set to 15.0% or more to less than 28.0%. The Cr content is preferably 17.0% or more, and more preferably 19.0% or more. Further, the Cr content is preferably 26.0% or less, and more preferably 24.0% or less.

Mo: 12.0% or less

W: less than 4.0%

Either of Mo (molybdenum) or W (tungsten) is an element which is dissolved in the austenitic structure forming a matrix, thus contributing to enhancing creep strength at a high temperature. To obtain such an advantageous effect, one or both of Mo and W are required to be contained. However, when contents of these elements are excessively increased, stability of the austenite phase is lowered on the contrary, thus causing lowering of creep strength. Accordingly, the Mo content is set to 12.0% or less. The Mo content is preferably 10.0% or less.

Further, the atomic weight of W is larger than the atomic weight of Mo. Accordingly, to obtain substantially the same advantageous effect as Mo, W is required to be contained with an amount larger than that of Mo and hence, W is disadvantageous in terms of cost and ensuring stability of the phase. Accordingly, the W content is set to less than 4.0%. Mo and W are not required to be contained in combination. When Mo or W is contained in a single form, either one of the Mo content or the W content is preferably 0.1% or more.

B: 0.0005 to 0.006%

B (boron) segregates at grain boundaries during use, thus strengthening the grain boundaries, and causing carbide at the grain boundaries to be finely dispersed and hence, creep strength is enhanced. Accordingly, B is an element necessary for enhancing creep strength. In addition to the above, B segregates at the grain boundaries, thus enhancing a sticking force and hence, B also has an effect of contributing to the improvement of toughness. To obtain these advantageous effects, it is necessary to set a content of B to 0.0005% or more. However, when B content is increased, and exceeds particularly 0.006%, a large amount of B segregates in the high-temperature HAZ near the fusion boundary due to welding heat cycle during welding and hence, B lowers the fusing point of the grain boundaries in an overlapping manner with P, thus increasing liquation cracking susceptibility in the HAZ. Accordingly, the B content is set to 0.0005 to 0.006%. The B content is preferably 0.001% or more, and preferably 0.005% or less.

Al: 0 to 3.0%

Ti: 0 to 3.0%

Nb: 0 to 3.0%

Any of Al (aluminum), Ti (titanium) or Nb (niobium) is an element which is bonded to Ni, and finely precipitates as intermetallic compound within grains, thus enhancing creep strength at a high temperature. However, when content of each element is excessively increased, and exceeds 3.0%, the effect is saturated, and creep ductility and toughness after long time heating are lowered. Accordingly, a content of each of Al, Ti, and Nb is set to 3.0% or less. The content of each element is preferably 2.8% or less, and more preferably 2.5% or less.

REM: 0 to 0.1%

Rare earth metal (REM) has strong affinity to P, and forms compound combined with P which has high fusing point and is stable even at a high temperature. Accordingly, REM has an action of fixing P, thus removing adverse effects of P on liquation cracking and toughness in the HAZ. REM is also an element which precipitates as carbide, thus contributing to enhancing high temperature strength. Accordingly, REM may be contained when necessary. However, when a content of REM is excessively increased, and exceeds 0.1%, in addition to that the effect of reducing adverse effects caused by P is saturated, a large amount of REM precipitates as carbides, thus causing lowering of toughness on the contrary. Accordingly, the REM content is set to 0.1% or less. The REM content is preferably 0.08% or less, and more preferably 0.06% or less. To obtain the advantageous effects, the REM content is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.01% or more.

REM indicates 17 elements in total, including Sc, Y, and the lanthanoids. The REM content means the total content of these elements.

Mg: 0 to 0.02%

Mg (magnesium) has strong affinity to S, and has an action of increasing hot workability. Mg also has an action of reducing both of generation of liquation cracking and lowering of toughness, which are caused by S, in the HAZ. Accordingly, Mg may be contained when necessary. However, excessive addition of Mg causes lowering of cleanliness due to bonding to oxygen. Particularly when a content of Mg exceeds 0.02%, cleanliness is remarkably lowered, thus deteriorating hot workability on the contrary. Accordingly, the Mg content is set to 0.02% or less. The Mg content is preferably 0.01% or less. However, to obtain the advantageous effects, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.001% or more.

Ca: 0 to 0.02%

Ca (calcium) has strong affinity to S, and has an action of increasing hot workability. Ca also has an action of reducing both of generation of liquation cracking and lowering of toughness, which are caused by S, in the HAZ. Accordingly, Ca may be contained when necessary. However, excessive addition of Ca causes lowering of cleanliness due to bonding to oxygen. Particularly when a content of Ca exceeds 0.02%, cleanliness is remarkably lowered, thus deteriorating hot workability on the contrary. Accordingly, the Ca content is set to 0.02% or less. The Ca content is preferably 0.01% or less. However, to obtain the advantageous effects, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.001% or more.

With respect to the alloy according to the present invention, it is necessary that the following formulas (i) to (iii) are satisfied in addition to that content of each element falls within the range. Symbol of an element in the following formulas (i) to (iii) refers to content (mass %) of each element.

0.1≤Mo+W≤12.0   (i)

As described above, either of Mo or W is an element which is dissolved in the austenitic structure forming a matrix, thus contributing to enhancing creep strength at a high temperature. However, when the contents of these elements are excessively increased, stability of the austenite phase is lowered on the contrary, thus causing lowering of creep strength. Accordingly, the total content of Mo and W is required to satisfy the formula (i). The value of the middle side in the formula (i) is preferably 1.0 or more to 10.0 or less.

1.0≤4×Al+2 ×Ti+Nb≤12.0   (ii)

To ensure favorable creep strength at a high temperature and toughness after long time heating by causing intermetallic compound bonded to Ni to finely precipitate within grains, it is necessary that one or more kinds selected from a group consisting of Al, Ti and Nb are contained, and the contents of such elements satisfy the formula (ii). The value of the middle side in the formula (ii) is preferably 3.0 or more to 11.0 or less.

P+0.2×Cr×B<0.035   (iii)

Either of P or B is an element which segregates at grain boundaries of the HAZ near the fusion boundary due to heat cycle during welding, thus lowering the fusing point and increasing liquation cracking susceptibility in the HAZ. During long-term use, although P segregated at the grain boundaries lowers sticking force of the grain boundaries, B strengthens the grain boundaries on the contrary. Accordingly, P adversely affects toughness, but B reduces the lowering of toughness on the contrary. Further, Cr is an element which affects segregation behavior of P and B at the grain boundaries, and indirectly affects properties of P and B. That is, the degree of effect of B on liquation cracking in the HAZ becomes more conspicuous with larger Cr content. Further, toughness in the HAZ after long-term use is significantly and adversely affected by P. However, in the case where the P content and the B content are substantially equal to each other, there is a tendency that toughness is lowered more with smaller Cr content.

To control grain-boundary segregation of P and B in the HAZ, thus obtaining excellent liquation cracking resistance and reducing the lowering of toughness after long time heating, the formula (iii) is required to be satisfied. The value of the left side in the formula (iii) is preferably 0.030 or less. The lower limit of the value of the left side in the formula (iii) is not particularly limited. However, the lower limit of the value of the left side may be set to a value close to 0.0015 which can be obtained when the content of P as an impurity is extremely low, Cr is 15.0%, and B is 0.0005%.

In the chemical composition of the Ni-based heat resistant alloy of the present invention, the balance consists of Fe and impurities. In this embodiment, “impurity” means a component which is mixed in industrially producing the alloy due to various causes, such as raw materials including ores or scrap, or production steps, and which is allowed to be mixed without adversely affecting the present invention.

2. Grain Size

Austenite grain size number at outer surface portion: −2.0 to 4.0

When an austenitic grain size at an outer surface portion is extremely large, 0.2% proof stress and tensile strength at a normal temperature are lowered. On the other hand, when an austenitic grain size at an outer surface portion is extremely small, it becomes impossible to maintain high creep rupture strength at a high temperature. Accordingly, the austenite grain size number at the outer surface portion is set to a value ranging from −2.0 to 4.0.

In the present invention, the grain size number is determined based on crossing line segments (grain size) defined by JIS G 0551 (2013). In a production process for a Ni-based alloy, by properly adjusting a heat-treatment temperature and holding time after hot working and a cooling method, it is possible to set the grain size number at the outer surface portion to a value which falls within the range after final heat treatment.

3. Size

Shortest distance from center portion to outer surface portion: 40 mm or more

As described above, in a large-sized structural member, in addition to a problem that 0.2% proof stress and tensile strength at a normal temperature are lowered, there is also a problem that creep rupture strength varies from region to region. However, the Ni-based heat resistant alloy according to the present invention exhibits sufficient 0.2% proof stress and tensile strength at a normal temperature, and sufficient creep rupture strength at a high temperature in large-sized structural members. That is, the present invention can obtain remarkable advantageous effects in members having a thick wall.

Accordingly, in the Ni-based heat resistant alloy of the present invention, the shortest distance from the center portion to the outer surface portion of a cross section is set to 40 mm or more, the cross section being perpendicular to a longitudinal direction. To obtain more remarkable advantageous effects of the present invention, the shortest distance from the center portion to the outer surface portion is preferably 80 mm or more, and more preferably 100 mm or more. In this embodiment, the shortest distance from the center portion to the outer surface portion refers to a radius (mm) of a cross section when an alloy has a columnar shape, and the shortest distance refers to a half-length (mm) of the short side of a cross section when an alloy has a quadrangular prism shape, for example.

As described later, the heat resistant alloy according to the present invention is obtained by performing hot working, such as hot forging or hot rolling on an ingot, or a cast piece, obtained by continuous casting or the like, for example. When an ingot is used, the longitudinal direction of a heat resistant alloy substantially refers to a direction along which a top portion and a bottom portion of the ingot are connected. When a cast piece is used, the longitudinal direction of a heat resistant alloy substantially refers to the longitudinal direction of the cast piece.

4. Precipitation Amount of γ′ Phase Obtained by Extraction Residue Analysis

(Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS)≤10.0   (iv)

where meaning of each symbol in the formula (iv) is as follows:

(Al+Ti+Nb)_(PB): total content of Al, Ti and Nb which are present at center portion as precipitates obtained by extraction residue analysis

(Al+Ti+Nb)_(PS): total content of Al, Ti and Nb which are present at outer surface portion as precipitates obtained by extraction residue analysis.

In a production process for an alloy, after heat treatment, which is performed after the hot working, is performed, undissolved γ′ phase (Ni₃ (Al, Ti, Nb)) is generated mainly within grains. Particularly at the center portion of the alloy, a cooling speed is slower than that at the outer surface portion of the alloy and hence, the amount of undissolved γ′ phase tends to increase. Accordingly, when the precipitation amount of Al, Ti and Nb which precipitate as γ′ at the center portion of the alloy increases compared with that at the outer surface portion of the alloy, and the value of (Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS) exceeds 10.0, it becomes impossible to maintain high creep rupture strength at a high temperature. On the other hand, it is not necessary to set the lower limit value of (Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS). However, there is a tendency that the amount of precipitates increases more at the center portion than at the outer surface portion and hence, (Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS) is preferably set to 1.0 or more.

The precipitate obtained by the extraction residue analysis is undissolved γ′ phase contained in the alloy. The extraction residue analysis is performed by the following procedure. First, test coupons for measuring γ′ phase are obtained from the center portion and the outer surface portion of the cross section of an alloy specimen, the cross section being perpendicular to the longitudinal direction of the alloy specimen. The surface area of each test coupon is obtained and, thereafter, only the base metal of the heat resistant alloy is completely electrolyzed in a 1% tartaric acid —1% ammonium sulfate aqueous solution under an electrolysis condition of 20 mA/cm². Then, the solution after electrolysis is performed is filtered through a 0.2 μm filter to extract precipitates as a residue. Thereafter, the extracted residue is decomposed with an acid, and is subjected to ICP-AES measurement to measure contents (mass %) of Al, Ti and Nb contained as undissolved γ′ phase and, then, the value of (Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS) is obtained based on the measured values.

5. Mechanical Properties

YS_(S)/YS_(B)31.5   (v)

TS_(S)/TS_(B)≤1.2   (vi)

where meaning of each symbol in the formulas is as follows:

YS_(B): 0.2% proof stress at center portion

YS_(S): 0.2% proof stress at outer surface portion

TS_(B): tensile strength at center portion

TS_(S): tensile strength at outer surface portion

In a large-sized structural member, a cooling speed at the time of performing heat treatment varies from region to region and hence, there is a tendency that great variations occur in mechanical properties from region to region due to the difference in the cooling speed. If there is a large difference in 0.2% proof stress and tensile strength at a normal temperature between the center portion and the outer surface portion of the large-sized structural member, there arises a problem that some regions do not satisfy the specifications. Accordingly, with respect to the Ni-based heat resistant alloy according to the present invention, mechanical properties at a normal temperature satisfy the formula (v) and formula (vi). It is not necessary to set the respective lower limit values of these formulas. However, there is a tendency that mechanical characteristics at the center portion are inferior to mechanical characteristics at the outer surface portion and hence, either one of formula (v) or formula (vi) is preferably set to 1.0 or more.

0.2% proof stress and tensile strength are obtained in such a way that round bar tensile test coupons, each having a parallel portion with a length of 40 mm, are cut out by mechanical processing from the center portion and the outer surface portion of the alloy parallel to the longitudinal direction, and a tensile test is performed on these test coupons at a room temperature. The tensile test is performed in accordance with JIS Z 2241 (2011).

6. Creep Rupture Strength

The Ni-based heat resistant alloy of the present invention is used in a high temperature environment, thus being required to be excellent in high temperature strength, particularly, in creep rupture strength. Accordingly, it is necessary that 10,000-hour creep rupture strength at 700° C. in the longitudinal direction is 150 MPa or more at the center portion of the alloy of the present invention.

Creep rupture strength is obtained by the following method. First, round bar creep rupture test coupons, described in JIS Z 2241 (2011), and having a diameter of 6 mm and a gage length of 30 mm, are cut out by mechanical processing from the center portions of the alloys parallel to the longitudinal direction. Then, a creep rupture test is performed in the atmosphere of 700° C., 750° C., and 800° C. to obtain 10,000-hour creep rupture strength at 700° C. by a Larson-Miller parameter method. The creep rupture test is performed in accordance with JIS Z 2271 (2010).

7. Production Method

The Ni-based heat resistant alloy of the present invention is produced by performing hot working on an ingot or a cast piece having the chemical composition. In the above step of performing hot working, processing is performed such that the longitudinal direction of the alloy in the final shape aligns with the longitudinal direction of the ingot or the cast piece forming a starting material. Hot working may be performed only in the longitudinal direction. However, to obtain a more uniform micro-structure at a higher working ratio, hot working may be performed one or more times in a direction substantially perpendicular to the longitudinal direction. After the hot working is performed, hot working of another method, such as hot extrusion, may be further performed when necessary.

In producing the Ni-based heat resistant alloy of the present invention, after the above step, final heat treatment described below is performed so as to minimize variation in metal micro-structure and mechanical properties from region to region, thus maintaining high creep rupture strength.

First, the alloy on which hot working was performed is heated to a heat-treatment temperature T (° C.) ranging from 1070 to 1220° C., and is held for 1150 D/T to 1500 D/T (min) within such a range. In this embodiment, symbol “D” denotes the diameter (mm) of the alloy when the alloy has a columnar shape, and “D” denotes a diagonal distance (mm) when the alloy has a quadrangular prism shape, for example. That is, symbol “D” denotes the maximum value (mm) of a linear distance between an arbitrary point on the outer edge of the cross section of the alloy and another arbitrary point on the outer edge, the cross section being perpendicular to a longitudinal direction of the alloy.

When the heat-treatment temperature is less than 1070° C., the amount of undissolved γ′ phase increases, thus lowering creep rupture strength. On the other hand, when the heat-treatment temperature exceeds 1220° C., grain boundaries are dissolved or grains are remarkably coarsened so that ductility is lowered. Accordingly, it is more desirable to set the heat-treatment temperature to 1100° C. or above, and it is more preferable to set the heat-treatment temperature to 1200° C. or below. Further, when the holding time is less than 1150 D/T (min), γ′ phase at the center portion increases and hence, (Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS) falls outside a range defined by the present invention. On the other hand, when the holding time exceeds 1500 D/T (min), grain at the outer surface portion is coarsened so that the austenite grain size number falls outside the range defined by the present invention.

Immediately after the alloy is heated and held, the alloy is cooled with water. This is because when a cooling speed becomes lower, particularly at the center portion of the alloy, a large amount of undissolved γ′ phase is generated mainly within grains so that there is a possibility that the formula (iv) is not satisfied.

Hereinafter, the present invention is described more specifically with reference to examples. However, the present invention is not limited to these examples.

EXAMPLE

Alloys having the chemical compositions shown in Table 1 were melted in a high-frequency vacuum furnace to prepare ingots each having an outer diameter of 550 mm, and a weight of 3t.

TABLE 1 Chemical composition (in mass %, balance: Fe and impurities) Alloy C Si Mn P S N O Ni Co Cr Mo W B 1 0.068 0.25 0.18 0.007 0.001 0.014 0.001 53.4 13.1 21.5 9.1 0.2 0.0031 2 0.052 0.14 0.24 0.005 0.001 0.008 0.002 49.5 20.5 20.3 5.1 1.5 0.0025 3 0.045 0.94 1.47 0.015 0.001 0.011 0.002 56.6 2.7 21.1 11.2 — 0.0015 4 0.024 0.58 0.33 0.012 0.001 0.012 0.001 54.1 8.1 25.6 6.8 — 0.0018 5 0.081 0.47 0.55 0.008 0.002 0.009 0.001 58.1 12.5 18.1 5.5 2.1 0.0054 6 0.110 0.62 0.86 0.008 0.001 0.010 0.001 52.5 10.7 22.4 8.3 0.4 0.0035 7 0.035 1.23 0.59 0.010 0.001 0.007 0.001 48.4 13.1 21.7 9.4 2.0 0.0043 8 0.042 0.20 0.73 0.007 0.001 0.009 0.001 43.6 23.6 17.4 10.6 0.3 0.0047 A 0.066 0.25 0.20 0.007 0.001 0.012 0.001 53.6 12.9 21.3 9.0 0.2 0.0029 B 0.068 0.24 0.21 0.008 0.001 0.013 0.001 53.4 13.2 21.4 8.9 0.3 0.0030 C 0.053 0.16 0.22 0.005 0.001 0.008 0.002 49.6 20.4 20.5 4.9 1.4 0.0025 D 0.054 0.14 0.22 0.006 0.001 0.009 0.002 49.5 20.5 20.5 4.9 1.5 0.0024 E 0.053 0.13 0.23 0.005 0.001 0.008 0.002 49.3 20.6 20.3 5.0 1.5 0.0025 F 0.069 0.23 0.21 0.006 0.001 0.012 0.001 53.3 12.9 21.6 1.4  8.2* 0.0030 G 0.050 0.15 0.22 0.006 0.001 0.009 0.001 45.6 20.1 18.1 9.4 3.8 0.0022 H 0.053 0.17 0.20 0.005 0.001 0.010 0.001 50.4 21.4 20.1 5.4 1.2 0.0028 Chemical composition (in mass %, balance: Fe and Middle side impurities) value Middle side value Left side value Alloy Al Ti Nb Others of formula (i)^(#1) of formula (ii)^(#2) of formula(iii)^(#3) 1 1.34 0.36 0.15 — 9.3 6.2 0.020 2 0.33 1.90 0.18 — 6.6 5.3 0.015 3 2.10 1.12 — REM: 0.023 11.2 10.6 0.021 4 1.52 0.24 0.15 Ca: 0.003 6.8 6.7 0.021 5 1.20 1.17 0.05 Mg: 0.005 7.6 7.2 0.028 6 1.71 — — — 8.7 6.8 0.024 7 — 2.61 — — 11.4 5.2 0.029 8 — — 2.85 — 10.9 2.9 0.023 A 1.32 0.37 0.14 — 9.2 6.2 0.019 B 1.33 0.36 0.14 — 9.2 6.2 0.021 C 0.35 1.85 0.16 — 6.3 5.3 0.015 D 0.35 1.87 0.18 — 6.4 5.3 0.016 E 0.31 1.89 0.17 — 6.5 5.2 0.015 F 1.36 0.38 0.15 — 9.6 6.4 0.019 G 0.32 1.85 0.20 — 13.2* 5.2 0.014 H 0.05 0.24 0.11 — 6.6 0.8* 0.016 *indicates that conditions do not satisfy those defined by the present invention. ^(#1)0.1 ≤ Mo + W ≤ 12.0 . . . (i) ^(#2)1.0 ≤ 4 × Al + 2 × Ti + Nb ≤ 12.0 . . . (ii) ^(#3)P + 0.2 × Cr × B < 0.035 . . . (iii)

The obtained ingots were processed to have a columnar shape with an outer diameter of 200 to 480 mm by hot forging, and final heat treatment was performed under conditions shown in Table 2 to obtain alloy member specimens. Alloys 1, 2, 3 and 5 were subjected to forging in a direction substantially perpendicular to the longitudinal direction after hot forging in the longitudinal direction and before final heat treatment and, thereafter, final hot forging was further performed in the longitudinal direction.

TABLE 2 Outer diameter Heat-treatment Holding time Alloy D (mm) temperature T (° C.) 1150D/T 1500D/T (min) Cooling method 1 450 1170 442 577 490 water cooling 2 350 1200 335 438 380 water cooling 3 430 1190 416 542 510 water cooling 4 200 1150 200 261 220 water cooling 5 480 1150 480 626 590 water cooling 6 500 1160 496 647 600 water cooling 7 650 1180 633 826 750 water cooling 8 200 1180 195 254 230 water cooling A 450 1170 442 577  650** water cooling B 450 1170 442 577  320** water cooling C 350  1020** 395 515 410 water cooling D 350  1250** 322 420 380 water cooling E 350 1200 335 438 380 air cooling** F 400 1200 383 500 440 water cooling G 370 1180 361 470 400 water cooling H 350 1170 344 449 390 water cooling **indicates that production conditions do not satisfy those defined by the present invention.

A test coupon for observing micro-structure was obtained from the outer surface portion of each specimen, and the cross section in the longitudinal direction was polished with emery paper and a buff. Thereafter, the test coupon was etched with a mixed acid, and optical microscopic observation was performed. The grain size number on an observation surface was obtained in accordance with a determination method defined by JIS G 0551 (2013) where the grain size number is determined based on crossing line segments (grain size).

Next, test coupons for measuring the amount of γ′ phase were obtained from the center portion and the outer surface portion of the cross section of each specimen, the cross section being perpendicular to the longitudinal direction of the specimen. The surface area of each test coupon was obtained and, thereafter, only the base metal of the heat resistant alloy was completely electrolyzed in a 1% tartaric acid—1% ammonium sulfate aqueous solution under an electrolysis condition of 20 mA/cm². Then, the solution after electrolysis was performed was filtered through a 0.2 μm filter to extract precipitates as a residue. Thereafter, extracted residue was decomposed with an acid, and was subjected to ICP-AES measurement to measure contents (mass %) of Al, Ti and Nb contained as undissolved γ′ phase and, then, the value of (Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS) was obtained based on the measured values.

Tensile test coupons, each having a parallel portion with a length of 40 mm, were cut out by mechanical processing from the center portion and the outer surface portion of each specimen parallel to the longitudinal direction, and a tensile test was performed on these test coupons at a room temperature so as to obtain 0.2% proof stress and tensile strength. Further, round bar creep rupture test coupon described in JIS Z 2241 (2011), and having a diameter of 6 mm and a gage length of 30 mm was cut out by mechanical processing from the center portion of each specimen parallel to the longitudinal direction. Then, a creep rupture test was performed in the atmosphere of 700° C., 750° C., and 800° C. to obtain 10,000-hour creep rupture strength at 700° C. by a Larson-Miller parameter method. These results are collectively shown in Table 3.

These results are collectively shown in Table 3.

TABLE 3 Grain size number at (Al + Ti + Nb)_(PB)/ Creep rupture Alloy outer surface portion (Al + Ti + Nb)_(PS) YS_(S)/YS_(B) TS_(S)/TS_(B) strength^(#4) 1 −1.2 7.3 1.2 1.1 161 Inventive 2 0.6 3.3 1.1 1.0 174 example 3 −1.6 6.5 1.3 1.1 179 4 1.4 5.4 1.2 1.1 160 5 0.7 7.8 1.1 1.1 165 6 0.4 7.3 1.3 1.1 172 7 −0.7 6.9 1.2 1.1 170 8 0.3 7.2 1.2 1.1 160 A −2.9* 6.7 1.7* 1.3* 155 Comparative B 3.4 11.4* 1.1 1.1 133 example C 5.7* 13.0* 1.2 1.1 134 D −3.1* 2.5 1.8* 1.3* 133 E 0.5 15.3* 1.2 1.1 134 F 0.4 8.1 1.2 1.1 145 G 0.5 8.4 1.2 1.1 143 H 0.9 7.8 1.1 1.0 129 *indicates that conditions fall outside the range of the present invention. ^(#4)indicates 10,000-hour creep rupture strengths at 700° C.

The alloys 1 to 8 are Inventive Examples of the present invention. The alloy composition, the grain size number, (Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS), YS_(S)/YS_(B), TS_(S)/TS_(B), and creep rupture strength of the alloys 1 to 8 fall within ranges defined by the present invention so that the alloys 1 to 8 have small variation in mechanical characteristics, and favorable creep rupture strength.

On the other hand, the alloy A and the alloy B have substantially the same chemical composition as the alloy 1, and are formed into a final shape same as that of the alloy 1 by hot forging. However, a holding time in heat treatment falls outside the production conditions defined by the present invention. Due to such holding time, the alloy A has the result that the grain size number at the outer surface portion falls outside the range defined by the present invention, and a value of YS_(S)/YS_(B) and a value of TS_(S)/TS_(B) fall outside the range defined by the present invention. Accordingly, the alloy A has a large variation in mechanical characteristics from region to region. Further, the alloy B falls outside the range defined by the present invention with respect to the value of (Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS) and, as a result, creep rupture strength of the alloy B is remarkably lower than that of the alloy 1.

Alloys C, D, and E have substantially the same chemical composition as the alloy 2, and are formed into a final shape same as that of the alloy 2 by hot forging. The alloy C is lower than the range defined by the present invention with respect to the heat-treatment temperature and hence, the value of (Al+Ti+Nb)/_(PB)/(Al+Ti+Nb)_(PS) and the grain size number at the outer surface portion fall outside the ranges defined by the present invention. As a result, creep rupture strength of the alloy C is remarkably lower than that of the alloy 2. The alloy D is higher than the range defined by the present invention with respect to a heat-treatment temperature and hence, the grain size number at the outer surface portion and a value of YS_(S)/YS_(B) and a value of TS_(S)/TS_(B) fall outside the range defined by the present invention. As a result, creep rupture strength of the alloy D is remarkably lower than that of the alloy 2. With regard to the alloy E, a cooling method in final heat treatment was not water cooling but was air cooling and hence, a cooling speed was remarkably low. Accordingly a value of (Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS) falls outside the range defined by the present invention and, as a result, creep rupture strength of the alloy E is remarkably lower than that of the alloy 3.

The alloys F, G, H are Comparative Examples where the chemical composition falls outside the specification of the present invention. To be more specific, the alloy F is an example where the W content is higher than the specification, the alloy G is an example where the value of the middle side in the formula (i) is higher than the specification, and the alloy H is an example where the value of the middle side in the formula (ii) is lower than the specification. Accordingly, as a result, creep rupture strength is low in these examples.

INDUSTRIAL APPLICABILITY

The Ni-based heat resistant alloy according to the present invention has small variation in mechanical properties from region to region, and is excellent in creep rupture strength at a high temperature. Accordingly, the Ni-based heat resistant alloy of the present invention is preferably applicable to a large-sized structural member for a boiler, a chemical plant or the like which is used in a high temperature environment. 

1. A Ni-based heat resistant alloy having a chemical composition consisting of, in mass %: C: 0.005 to 0.15%; Si: 2.0% or less; Mn: 3.0% or less; P: 0.030% or less; S: 0.010% or less; N: 0.030% or less; O: 0.030% or less; Ni: 40.0 to 60.0%; Co: 0.01 to 25.0%; Cr: 15.0% or more to less than 28.0%; Mo: 12.0% or less; W: less than 4.0%; B: 0.0005 to 0.006%; Al: 0 to 3.0%; Ti: 0 to 3.0%; Nb: 0 to 3.0%; REM: 0 to 0.1%; Mg: 0 to 0.02%; Ca: 0 to 0.02%; and the balance: Fe and impurities, wherein following formulas (i) to (iii) are satisfied, a shortest distance from a center portion to an outer surface portion of a cross section of the alloy is 40 mm or more, the cross section being perpendicular to a longitudinal direction of the alloy, an austenite grain size number at the outer surface portion is −2.0 to 4.0, a total content of Al, Ti and Nb which are present as precipitates obtained by extraction residue analysis satisfies a following formula (iv), and mechanical properties at a normal temperature satisfy a following formula (v) and a following formula (vi): 0.1≤Mo+W≤12.0   (i) 1.0≤4×Al+2 ×Ti+Nb≤12.0   (ii) P+0.2×Cr×B<0.035   (iii) (Al+Ti+Nb)_(PB)/(Al+Ti+Nb)_(PS)≤10.0   (iv) YS_(S)/YS_(B)31.5   (v) TS_(S)/TS_(B)≤1.2   (vi) wherein, symbol of an element in the formulas (i) to (iii) refers to content (mass %) of each element, and meaning of each symbol in the formulas (iv) to (vi) is as follows: (Al+Ti+Nb)_(PB): total content of Al, Ti and Nb which are present at center portion as precipitates obtained by extraction residue analysis (Al+Ti+Nb)_(PS): total content of Al, Ti and Nb which are present at outer surface portion as precipitates obtained by extraction residue analysis YS_(B): 0.2% proof stress at center portion YS_(S): 0.2% proof stress at outer surface portion TS_(B): tensile strength at center portion TS_(S): tensile strength at outer surface portion.
 2. The Ni-based heat resistant alloy according to claim 1, wherein the chemical composition comprises one or two elements selected from a group consisting of, in mass %: Mg: 0.0001 to 0.02%; and Ca: 0.0001 to 0.02%.
 3. The Ni-based heat resistant alloy according to claim 1, wherein 10,000-hour creep rupture strength at 700° C. in the longitudinal direction at the center portion is 150 MPa or more.
 4. A method for producing a Ni-based heat resistant alloy, the method comprising the steps of: performing hot working on an ingot or a cast piece having the chemical composition according to claim 1; and thereafter performing heat treatment where the ingot or the cast piece is heated to a heat-treatment temperature T (° C.) ranging from 1070 to 1220° C., is held for 1150 D/T to 1500 D/T (min), and is cooled with water, wherein symbol “D” denotes a maximum value (mm) of a linear distance between an arbitrary point on an outer edge of a cross section of the alloy and another arbitrary point on the outer edge, the cross section being perpendicular to a longitudinal direction of the alloy.
 5. The method for producing a Ni-based heat resistant alloy according to claim 4, wherein in the step of performing the hot working, the hot working is performed one or more times in a direction substantially perpendicular to a longitudinal direction in the hot working.
 6. The Ni-based heat resistant alloy according to claim 2, wherein 10,000-hour creep rupture strength at 700° C. in the longitudinal direction at the center portion is 150 MPa or more.
 7. A method for producing a Ni-based heat resistant alloy, the method comprising the steps of: performing hot working on an ingot or a cast piece having the chemical composition according to claim 2; and thereafter performing heat treatment where the ingot or the cast piece is heated to a heat-treatment temperature T (° C.) ranging from 1070 to 1220° C., is held for 1150 D/T to 1500 D/T (min), and is cooled with water, wherein symbol “D” denotes a maximum value (mm) of a linear distance between an arbitrary point on an outer edge of a cross section of the alloy and another arbitrary point on the outer edge, the cross section being perpendicular to a longitudinal direction of the alloy.
 8. The method for producing a Ni-based heat resistant alloy according to claim 7, wherein in the step of performing the hot working, the hot working is performed one or more times in a direction substantially perpendicular to a longitudinal direction in the hot working. 